Resonantly driven fiber polarization scrambler

ABSTRACT

The invention provides a polarization scrambler to rapidly change the state of polarization of light transmitted through a single mode fiber such that a substantial portion of the Poincare sphere is covered. A plurality of piezoelectric squeezers couple with an electronic drive. Each squeezer resonates in response to drive signals from the electronic drive to induce radial compression forces on the fiber. The drive signals remain resonant with each squeezer, at frequencies of about 100 kHz, through a feedback loop for each of the squeezers and the electronic drives. Coverage over the Poincare sphere typically occurs in less than about 100 milliseconds, and preferably less than 1 millisecond.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application SerialNo. 60/296,839, filed Jun. 8, 2001, entitled, “Resonantly Driven FiberPolarization Scrambler and is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Polarization scramblers are used in many applications relating tofiber-optic test and measurement. These scramblers typically utilizepiezoelectric transducers (sometimes known as “squeezers”) and provideincreasing benefit with increasing speed and/or efficiency in completelygenerating a plurality of states of polarization (SOP) that cover asubstantial portion of the Poincare sphere of polarization states.Polarization scramblers in the prior art use inefficient, high-voltagetechniques to drive the piezoelectric transducers. Such techniques haveseveral disadvantages; first and foremost, the high voltage itself is adrawback. Second, when driving a capacitive load (i.e., thepiezoelectric transducers) with high voltage, the scrambler's speed ofoperation decreases due to bandwidth limitations.

The invention circumvents the afore-mentioned problems, in one object,by driving the piezoelectric transducers at resonant frequencies,resulting in higher-speed operation at lower voltages, as compared tothe prior art. Other objects of the invention are apparent within thedescription that follows.

SUMMARY OF THE INVENTION

The following patents provide useful background information for theinvention: U.S. Pat. Nos. 5,682,445; 5,633,959; 5,408,545; 5,159,481;4,988,169; 4,979,235; 4,960,319; 4,923,290; 4,753,507; 4,753,507;3,645,603; and 3,625,589. Each of the afore-mentioned patents isexpressly incorporated herein by reference.

The following articles provide useful background information for theinvention: M. Johnson, In-line fiber-optical polarization transformer,Appl. Opt. 18, p.1288 (1979); R. Ulrich, Polarization stabilization onsingle-mode fiber, Appl. Phys. Lett. 35, p. 840 (1979); Kidoh et al.,Polarization control on ouptut of single-mode optical fibers, IEEE J.Quan. Elec. 17, p. 991 (1981); R. Alferness, Electrooptic guided-wavedevice for general polarization transformations, IEEE J. Quan. Elec. 17,p.965 (1981); Sakai et al., Birefringence and polarizationcharacteristics of single-mode optical fibers under elasticdeformations, IEEE J. Quan. Elec. 17, p.1041 (1981); R. Noe, Endlesspolarization control in coherent optical communications, Elec. Lett. 22,p.772 (1986); R. Noe, Endless polarization control experiment with threeelements of limited birefringence range, Elec. Lett. 22, p.1341 (1986);N. Walker et al., Endless polarization control using four fibersqueezers, Elec. Lett. 23, p. 290 (1987); A. Kersey et al., Monomodefiber polarization scrambler, Elec. Lett. 23, p.634 (1987); Tatam etal., Full polarization state control utilizing linearly birefringentmonomode optical fiber, IEEE J. Lightwave Tech. 5, p.980 (1987); GWalker et al., Rugged, all-fiber, endless polarization controller, Elec.Lett. 24, p.1353 (1988); 2×2 Optical Fiber Polarization Switch andPolarization controller, Elec. Lett. 24, p.1427 (1988); and S. Siddiqui,Liquid crystal polarization controller for use in fiber communicationsystems, Optical Fiber Conference Proceedings, Wed. afternoon, poster#122 (1989). Each of the afore-mentioned articles is incorporated hereinby reference.

This invention of one aspect provides a fiber-based polarizationscrambler. The scrambler uses multiple piezoelectric squeezers thatexert radial forces on a section of single-mode optical fiber. Theradial forces on the fiber change the fiber's birefringence via thephotoelastic effect, which changes the SOP of light transmitted throughthe section of squeezed fiber. In the preferred aspect of the invention,each of the piezoelectric squeezers is excited independently at one ofits resonant frequencies by an electronic drive. Each squeezer may bedriven at the same or different frequencies from every other squeezer.Use of resonant frequencies in the polarization scrambler of theinvention thus reduces the drive voltages required to change the SOP,and yet provides for higher speed and efficiency, as compared to priorart polarization scramblers.

In another aspect of the invention, the electronic drive controlsamplitude output independently from drive frequency, for each squeezer.In a related aspect, the electronic drive controls amplitude outputsignals substantially independently from drive frequency, for eachsqueezer.

In still another aspect, the polarization scrambler of the inventioncreates a plurality of polarization states—sometimes referred to ascoverage of the Poincare sphere. Preferably, the plurality ofpolarization states includes all polarization states to coversubstantially all of the Poincare sphere. One measure of theeffectiveness of the scrambler is the Degree of Polarization (DOP),defined in Born et al., Principles of Optics, 6th Edition, PergamonPress, p.554-555 (1980):${D\quad O\quad P} = \frac{\sqrt{{\langle S_{1}\rangle}^{2} + {\langle S_{2}\rangle}^{2} + {\langle S_{3}\rangle}^{2}}}{S_{0}}$

where the S_(i) are components of the Stokes vector that describes theSOP at a given moment in time, and the <> indicate the average of theStokes component over a measurement time-interval. Minimizing DOP over ameasurement time interval (1/measurement bandwidth) depends ongenerating a plurality of polarization states within the time they aremeasured. In one aspect of the invention, the plurality of polarizationstates are produced in a time less than about 100 milliseconds, andpreferably less than 1 millisecond. The DOP will be less than about 5%at a measurement bandwidth of 10 kHz. In a further aspect, the scramblergenerates a pattern or sequence of polarization states; these patternsor sequences may be random or periodic in time.

The invention of one aspect provides an improvement to a polarizationsensitive optical measurement system. Such a system can include, forexample, polarization-dependent loss devices orpolarization-mode-dispersion measuring devices. In accord with theinvention, the optical measurement system incorporates a polarizationscrambler, such as described above, to quickly and efficiently inducecoverage across the Poincare sphere, detuning or eliminating systemsensitivity to polarization effects. Those skilled in the art shouldappreciate that the improvement provided by the polarization scramblerof the invention can provide further enhancements to other fiber opticalsystems and instruments of the prior art.

In yet another aspect, the invention provides an improvement to anoptical source. Such a source can include, for example, a laser diode,LED, or amplified spontaneous emission devices.

A key consideration for users of polarization scramblers within thefiber optic marketplace relates to speed of operation; since theinvention provides improved operating speed over prior art polarizationscramblers, the invention provides obvious advantages.

The invention is next described further in connection with preferredembodiments, and it will become apparent that various additions,subtractions, and modifications can be made by those skilled in the artwithout departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be obtained byreference to the drawings, in which:

FIG. 1 shows a polarization scrambler constructed according to theinvention;

FIG. 2 shows a serial array of fiber squeezers applying radialcompression forces onto a fiber, according to the invention;

FIG. 3 shows an end view of the squeezers of FIG. 2;

FIG. 4 schematically illustrates one circuit suitable for driving apiezoelectric squeezer in accord with the invention;

FIG. 5 shows a polarization scrambler optionally arranged in theimprovement of optical sources, in accord with the invention; and

FIG. 6 shows a polarization scrambler optionally arranged in theimprovement of polarization sensitive optical systems or devices, inaccord with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a fiber-based polarization scrambler 10 (not to scale)constructed according to the invention. Scrambler 10 includes aplurality of piezoelectric squeezers 12 arranged such that each squeezerproduces a radial force, selectively, onto an optical fiber 14 passingthrough scrambler 10. An electric drive 16 electrically connects to eachsqueezer 12, via corresponding electrical pathways 18, and applies aresonant voltage across each squeezer 12 in a pre-selected magnitude,frequency and duty cycle, as described herein.

Scrambler 10 shows three squeezers 12 a, 12 b, 12 c and threecorresponding sets of electrical pathways 18 a, 18 b, 18 c connectingsqueezers 12 a, 12 b, 12 c, respectively, to electronic drive 16. Thoseskilled in the art should appreciate that more or fewer squeezers 12 maybe implemented within scrambler 10 without departing from the scope ofthe invention. Those skilled in the art should also appreciate that thearrangement of, and the electrical connections 15 to and between,electronic drive 16, pathways 18 and squeezers 12 are shown for purposesof illustration, and not in a limiting way; and that a variety ofconnections and transducer arrangements may be made within the scope ofthe invention.

In the preferred embodiment of the invention, electronic drive 16excites each squeezer 12 at a resonant frequency. In that each squeezer12 has a plurality of resonant frequencies, the selected resonantfrequency of any one squeezer 12—as excited by electronic drive 16—maybe the same or different from the resonant frequency of any othersqueezer 12. A housing 24 illustratively surrounds squeezers 12 so as toleverage compression forces between squeezers 12 and fiber 14.Electronic drive 16 also preferably controls voltage amplitude appliedto each squeezer 12 independently from the drive frequency used toattain resonance. Essentially, it is desirable in most embodiments ofthe invention to drive each squeezer 12 through its full range ofphysical motion, as quickly as possible, to achieve the benefits ofpolarization scrambling according to the teachings herein.

In operation, application of force onto fiber 14 by any squeezer 12alters the polarization properties of fiber 14. Electronic drive 16excites the plurality of squeezers 12 in a manner so as to substantiallyeliminate polarization effects of electromagnetic energy 22 transmittedthrough fiber 14 to downstream electro-optical systems, devices orcomponents 22. Specifically, by driving squeezers 12 independently witha random or periodic pattern or sequence, fiber 14 experiences aplurality of polarization states to cover the Poincare sphere over ashort time interval, e.g., less than one hundred milliseconds, andpreferably less than one millisecond.

The design and construction of squeezer 12 may be made in accord withU.S. Pat. No. 4,753,507, incorporated herein by reference. Preferably, afixed angular relationship exists between any two adjacent squeezers 12,optimally at an angle of about forty-five degrees between any twoadjacent squeezers (e.g., squeezers 12 a, 12 b and squeezers 12 b, 12c). FIG. 2 illustrates three fiber squeezers 12′ constructed andarranged to independently apply radial compression forces 45 on singlemode fiber 14′. As shown in FIG. 3, squeezers 12′ apply these forcesonto fiber 14′ with circumferential offset angles (θ) between adjacentsqueezers of zero degrees, sixty degrees and minus sixty degrees. Otherangular arrangements of squeezers 12′ may also be made, in accord withthe invention, such as 0 degrees, forty-five degrees, and minusforty-five degrees. Squeezers 12′ thus represent three variableretarders, in series, along fiber 14′. Like FIG. 1, squeezers 12′scramble polarizations along the Poincare sphere. Squeezers 12′ may bemanually controlled to reach practically any polarization state.Squeezers 12′ can include piezoelectric transducers electrically excitedto apply compression forces 45, as shown.

Electronic drive 16 electrically excites squeezers 12 at a frequency ofat least about 100 kHz. Although lower frequencies may be used,frequencies such as 100 kHz permit high speed and efficiency in coveringthe Poincare sphere, and minimize the DOP at high bandwidths, such as 10kHz. Accordingly, electronic drive 16 utilizes a electronic feedbackloop to appropriately match an individual squeezer's mechanical orelectrical resonance. By way of example, as described in connection withFIG. 4, the feedback loop of drive 16 may include a phase-locked loop(“PLL”) with a voltage-controlled oscillator (“VCO”).

FIG. 4 shows a schematic of one electronic circuit 50 illustrating anelectrical and feedback relationship between drive 16 and one squeezer12 of FIG. 1. A VCO 52 is integral with a PLL 54 to produce anoscillatory voltage across piezoelectric squeezer 12. The frequency ofthe oscillation is chosen to be at or near to one of the mechanical orelectrical resonance frequencies of squeezer 12. The oscillatory voltageresponse of squeezer 12 is also measured by one of two alternativetechniques, both shown in FIG. 2 for purpose of illustration. In a firsttechnique, a sense voltage proportional to the squeezer's drive current,determined by current monitor circuit 55, is measured across a senseresistor 56 in series with squeezer 12. In a second technique, a voltagesense circuit 58 measures a voltage across squeezer 12. In eithertechnique, a voltage 74 indicative of oscillatory voltage of squeezer 12is fed to the phase comparator input 60 of PLL 54. The phase errorbetween the VCO drive voltage 78 at VCO Output 76 and voltage 74 atinput 60 is sent to loop filter 64, which integrates, phase shiftsand/or amplifies the error signal over a defined bandwidth. The output66 of loop filter 64 is summed at junction 68 with an adjustable DCvoltage 70, which provides a VCO center frequency adjustment to VCO In62 in the absence of an error signal 75. PLL 54 adjusts the frequencydifference until the phase error is zero, represented by phase errorsignal 75 at PLL Comparator Out 76. When the phase difference betweendrive voltage 78 and sense voltage 74 is at or near to zero, squeezer 12is driven at or near to resonance. VCO Out 76 provides the input to thefeedback section with squeezer 12; preferably, a high current (e.g.,less than or equal to 1A), intermediate voltage stage 80 electronicallycouples output 78 to squeezer 12.

PLL 54 may for example be a microchip of common design, including the 0or 90 degree feedback PLL. However, the PLL can also be constructedthrough discrete components, as known in the art. Those skilled in theart should also appreciate that the functions of PLL 54 may beimplemented with a frequency lock circuit, such as those usingmodulating/demodulating techniques, or with a self-resonant circuit. Thefeedback to PLL 54 may further derive from signals generated fromadditional squeezers 12 or other devices.

One practical concern is the maintaining of phase lock in the event thatthe squeezer resonance changes, such as caused through thermal ormechanical stresses. As known to those skilled in the art, passiveinsulation measures may be used to isolate the squeezer from theseexternal stresses during the feedback operation. Alternatively, activetemperature monitoring and control of squeezer 12 or other components54, 64, 55, 58 may help reduce resonance variations. Thermistors or RTDdevices may be used to monitor temperature; resistive heaters or Peltierdevices may be used to control temperature. Preferably, circuit 50includes circuit elements that decrease the resonant Q-factor, such thatthe squeezer is less sensitive to changes in the resonance frequency,without substantially altering frequency response. Optionally,therefore, circuit element Z1 is included to increase the damping ofcircuit 50; element Z1 can for example include inductors and resistorsin series. Circuit element Z2, representing an inductor in parallel withsqueezer 12, may also be optionally included to resonate at frequency(LC){circumflex over ( )}−½, defined by internal L, C components, whichis at or near to the mechanical resonance of squeezer 12; this furtherbroadens the resonant response of circuit 50 for a given amplitude.Alternatively, L and C may be chosen at frequencies far from amechanical resonance, thereby creating a purely electrical resonancewhose frequency and Q-factor are dictated by the impedances of thesqueezer and of elements Z1 and Z2. Those skilled in the art shouldappreciate that elements Z1, Z2 may include inductors, capacitors,resistors, transistors, op amps, diodes and/or other electricalcomponents as a matter of design choice to provide like functionality.

FIG. 5 illustrates how a polarization scrambler 100 of the invention maybe used in the improvement of an optical source 102. By way of example,an optical source 102 with a laser, diode or a spontaneous emissiondevice 104 is coupled with scrambler 100 via a single mode fiber 106.Scrambler 100 processes electromagnetic energy from driver 104, throughfiber 106, to produce electromagnetic energy 110 whose polarizationstate is substantially and rapidly modulated in time. Scrambler 100 maybe made integral with the housing 108 of source 102 to provide a compactoptical source for generating polarization-scrambled optical energy.

FIG. 6 illustrates how a polarization scrambler 120 of the invention maybe used in the improvement of a polarization-sensitive optical system122. By way of example, system 122 may include one or morepolarization-dependent loss or polarization-mode-dispersion devices 124.A fiber 126 accepts electromagnetic inputs to system 122, and scrambler120 processes the inputs to provide a substantiallypolarization-scrambled signal 128 to devices 124. As appropriate,scrambler 120 may be made within the housing 130 of system 122 so as toprovide a modular package for users of system 122.

The invention thus attains the objects set forth above, among thoseapparent from the preceding description. Since certain changes may bemade in the above methods and systems without departing from the scopeof the invention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawing be interpreted asillustrative and not in a limiting sense. It is also to be understoodthat the following claims are to cover all generic and specific featuresof the invention described herein, and all statements of the scope ofthe invention which, as a matter of language, might be said to fallthere between.

Having described the invention, what is claimed is:
 1. A polarizationscrambler for electromagnetic energy through an optical fiber,comprising: a plurality of piezoelectric squeezers and an electronicdrive, each squeezer resonating in response to drive signals from theelectronic drive to induce radial compression forces onto the fiber,wherein output polarization state of electromagnetic energy from thescrambler, averaged over a test time period of less than about 100milliseconds, is substantially non-polarized.
 2. A scrambler of claim 1,wherein at least one squeezer resonates at a mechanical resonancefrequency.
 3. A scrambler of claim 1, wherein at least one squeezeroscillates at a resonance frequency determined by the electronic driveand the squeezer.
 4. A scrambler of claim 1, wherein the plurality ofsqueezers comprise three squeezers arranged serially along a portion ofthe fiber, each squeezer applying the radial compression forces onto thefiber at a circumferential offset angle relative to any adjacentsqueezer.
 5. A scrambler of claim 4, wherein the circumferential offsetangle is 45 degrees.
 6. A scrambler of claim 4, wherein thecircumferential offset angle is 60 degrees.
 7. A scrambler of claim 1,wherein the electronic drive excites each squeezer with a controlled RMSvoltage amplitude at a selected frequency.
 8. A scrambler of claim 1,wherein the optical fiber comprises a single mode fiber.
 9. A scramblerof claim 1, wherein the optical fiber comprises a multi-mode fiber. 10.A scrambler of claim 1, wherein the electronic drive provides excitationsignals to at least one squeezer in a random duty cycle.
 11. A scramblerof claim 1, wherein the electronic drive provides excitation signals toat least one squeezer in a periodically varying duty cycle.
 12. Apolarization scrambler for electromagnetic energy through an opticalfiber, comprising: a plurality of piezoelectric squeezers having aresonant frequency of at least about 10 kHz, and an electronic drive,each squeezer resonating in response to drive signals from theelectronic drive to induce radial compression forces onto the fiber,wherein output polarization state of electromagnetic energy from thescrambler, averaged over a test time period, is substantiallynon-polarized.
 13. A polarization scrambler for electromagnetic energythrough an optical fiber, comprising: a plurality of piezoelectricsqueezers and an electronic drive, each squeezer resonating in responseto drive signals from the electronic drive to induce radial compressionforces onto the fiber, wherein the electronic drive and squeezerscooperate to invoke a plurality of polarization states thatsubstantially cover the Poincare sphere in less than about 1millisecond, and wherein output polarization state of electromagneticenergy from the scrambler, averaged over a test time period, issubstantially non-polarized.
 14. A polarization scrambler forelectromagnetic energy through an optical fiber comprising: a pluralityof piezoelectric squeezers and an electronic drive, each squeezerresonating in response to drive signals from the electronic drive toinduce radial compression forces onto the fiber, wherein the electronicdrive and squeezers cooperate to invoke a plurality of polarizationstates that substantially cover the Poincare sphere in less than about100 milliseconds, and wherein output polarizatiion state ofelectromagnetic energy from the scrambler, averaged over a test timeperiod, is substantially non-polarized.
 15. A polarization scrambler forelectromagnetic energy through an optical fiber, comprising: a pluralityof piezoelectric squeezers and an electronic drive, each squeezerresonating in response to drive signals from the electronic drive toinduce radial compression forces onto the fiber, wherein outputpolarization state of electromagnetic energy from the scrambler,averaged over a test time period of less than about 1 millisecond, issubstantially non-polarized.
 16. A polarization scrambler forelectromagnetic energy through an optical fiber, comprising: a pluralityof piezoelectric squeezers and an electronic drive, each squeezerresonating in response to drive signals from the electronic dirve toinduce radial compression forces onto the fiber, wherein the electronicdrive and squeezers cooperate to invoke a plurality of polarizationstates providing a degree of polarization, output from the scrambler,that is less than about 5% when measured over a test time period of lessthan about 100 msec.
 17. A polarization scrambler for electromagneticenergy through an optical fiber, comprising: a plurality ofpiezoelectric squeezers and an electronic drive, each squeezerresonating in response to drive signals from the electronic drive toinduce radial compression forces onto the fiber, wherein the electronicdrive and squeezers cooperate to invoke a plurality of polarizationstates providing a degree of polarization, output from the scrambler,that is less than about 5% when measured over a test time period of lessthan about 1 msec.
 18. A method of scrambling polarizations through afiber receiving compression forces from a plurality of piezoelectricsqueezers, comprising the steps of: generating one or more oscillatingsignals that induce vibrations in each of the squeezers, sensingfeedback signals representative of the vibrations, and coupling theoscillating signals with the feedback signals in a feedback control loopstabilizing vibrations of at least about 10 kHz in each of thesqueezers.
 19. A method of claim 18, wherein the step of generating oneor more oscillating signals comprises utilizing one or more voltagecontrolled oscillators.
 20. A method of claim 18, wherein the step ofcoupling the oscillating signals with the feedback signals comprisesutilizing a phase-locked loop.
 21. A method of claim 18, wherein thestep of coupling the oscillating signals with the feedback signalscomprises utilizing a frequency lock circuit.
 22. A method of claim 18,wherein the step of coupling the oscillating signals with the feedbacksignals comprises utilizing a self-resonant circuit.
 23. A method ofclaim 18, wherein the step of sensing feedback signals comprises sensingdrive current of each squeezer, measured across a resistor in serieswith the squeezer.
 24. A method of claim 18, wherein the step of sensingfeedback signals comprises determining a voltage across the squeezer.25. A method of claim 18, wherein the step of coupling the oscillatingsignals with the feedback signals comprises comparing phases of thesignals through a phased-lock loop, and further comprising generating anerror signal, indicative of phase error, to drive the feedback controlloop to stable resonance.
 26. A method of claim 25, further comprisingadding a DC voltage to the error signal to provide a VCO centerfrequency.
 27. In an optical source generating electromagnetic energy inan optical fiber, the improvement comprising: a polarization scramblerhaving a plurality of piezoelectric squeezers and an electronic drive,each squeezer resonating in response to drive signals from theelectronic drive to induce radial compression forces onto the fiber,wherein a polarization state of output electromagnetic energy from thesource is substantially non-polarized over a test time period less thanabout 100 milliseconds.
 28. In an optical source generatingelectromagnetic energy in an optical fiber, the improvement comprising:a polarization scrambler having a plurality of piezoelectric squeezersand an electronic drive, each squeezer resonating in response to drivesignals from the electronic drive to induce radial compression forcesonto the fiber, wherein a polarization state of output electromagneticenergy from the source is substantially non-polarized over a test timeperiod less than about 1 millisecond.
 29. In a source of claim 28, theimprovement comprising one of a laser, diode or spontaneous emissionsource for generating the energy.
 30. In a polarization sensitiveoptical system receiving electromagnetic energy from an optical fiber,the improvement comprising: one or more polarization sensitive devices,a polarization scrambler having a plurality of piezoelectric squeezers,and an electronic drive, each squeezer resonating in response to drivesignals from the electronic drive to induce radial compression forcesonto the fiber, wherein an output polarization state of theelectromagnetic energy coupled to the devices is substantiallynon-polarized over a test time period of less than about 100milliseconds.
 31. In an optical system of claim 30, the improvementwherein the test time period is less than about 1 millisecond.